Sputtering, alternatively called physical vapor deposition (PVD), has long been used in depositing metals and related materials in the fabrication of semiconductor integrated circuits. Its use has been extended to depositing metal layers onto the sidewalls of high aspect-ratio holes such as vias or other vertical interconnect structures. Currently, advanced sputtering applications include depositing a copper seed layer for later electroplating of copper in the via and depositing a barrier layer, such as tantalum and its nitride, on the dielectric material of the via sidewall to prevent the copper from diffusing into the dielectric.
Plasma sputtering typically includes a magnetron positioned at the back of the sputtering target to project a magnetic field into the processing space to increase the density of the plasma and enhance the sputtering rate. Typically, the magnetron is rotated about the center of the circular target to provide a more uniform erosion pattern of the target and deposition profile on the circular wafer.
Sputtering into high aspect-ratio vias has prompted further modifications in the magnetron which promotes the ionization of a large fraction of the sputtered atoms. If the wafer is electrically biased, the sputtered ions are accelerated in nearly vertical trajectories to reach deeply within the vias. The ionization fraction of sputtered atoms is increased if the magnetron is relatively small so that the target power is effectively concentrated in a small fraction of the target area adjacent the small magnetron. However, very small magnetrons rotating about a target center introduce two problems. For copper sputtering especially, target utilization and radial deposition uniformity are reduced if the magnetron is rotating along a fairly narrow annular band and promoting sputtering only within that band. For tantalum sputtering, it seems acceptable to sputter only the outer peripheral band of the target because the tantalum ions tend to diffuse toward the center during their passage to the wafer. However, some of the sputtered tantalum tends to redeposit on the target. In the unsputtered central area of the target, the redeposited tantalum, often in nitride form, forms a growing layer of poorly adhering material. Eventually, the redeposited material flakes off and create a significant particle problem.
Miller et al. in U.S. Pat. No. 6,852,202 describe a planetary magnetron executing a regular epicyclic pattern at the back of the target, thereby increasing the sputtering uniformity. An epicyclic pattern is obtained which combines rotation of the magnetron about an axis that is itself rotating about a central axis. In a regular epicyclic pattern the two rotation rates are constant such as the moon orbits the earth and the earth orbits the sun with two orbital periods with a constant proportionality between them. Typically, the rotation rates are constant over the entire active scan. On the other hand, a general epicyclic pattern can still be characterized as a two-stage rotation of the magnetron but the two orbital periods do not necessarily have a fixed ratio.
Rosenstein et al. in U.S. Pat. No. 6,228,236 and Pavloff in U.S. patent application Ser. No. 11/553,880, filed Oct. 27, 2006 and now published as U.S. patent application publication 2008/0099329 disclose magnetrons which rotate at two different radii when rotated in opposite directions. This dual operation allows sputtering depositing on the wafer at one magnetron radius and cleaning the target at a second magnetron radius while the wafer is removed from the chamber. The required reversal of rotation, however, is inconvenient. Gung et al. in U.S. patent application publication 2005/0211548 disclose a centrifugal mechanism for switching between two rotation radii dependent upon the rotation speed. Miller et al. in U.S. patent application publication 2006/007623, the parent application of this continuation in part and incorporated herein by reference, disclose mechanisms for the continuous variability of the magnetron radius.
In U.S. patent application publication 2005/1013365, Hong et al. have disclosed a mechanism for vertically moving a planetary magnetron to compensate for target erosion.